So called start/stop systems are becoming increasingly common in vehicles having internal combustion engines. These systems have a proven ability to provide fuel savings, especially during city driving. This is due to a shutoff of the internal combustion engine when not required to propel the vehicle and a restart thereof once required again.
Some start/stop vehicles have two batteries. When starting in a start/stop mode, i.e., a warm start following a shutoff of the internal combustion engine, e.g., during a stop at an intersection, one main battery supplies the starter motor while a second battery supplies the vehicle systems with electric current. However, when starting using a key, starter button or similar, i.e., so called cold start, the main battery supplies both the vehicle systems and the starter motor.
However, a problem associated with the supply of electric current to vehicle systems is that some of these systems comprise components which are critical for starting of the internal combustion engine of the vehicle, such as an Engine Control Unit (ECU)/Engine Control Module (ECM) and injector valves etc. Some of these start critical components require a voltage above a certain threshold voltage, e.g., of about 6V, in order to operate as required.
The battery performance, i.e., capability to keep the voltage above the threshold voltage at a specified current drain, is both due to State of Charge (SOC) and some ageing factors, e.g., the battery capacity, where the capacity is continuously reduced during the battery service life, and the battery internal resistance, which is increasing during battery service life.
When the main battery performance is reduced by any of above listed reasons, there is a range where the main battery power will suffice to enable the starter motor to crank the internal combustion engine. However, the main battery power may at the same time be insufficient to maintain the voltage at a sufficiently high level for keeping the ECU/ECM and possibly other start critical systems operating as required for starting.
In order to mitigate this problem of draining of a vehicle starter battery one known solution, according to U.S. Pat. No. 7,216,617, aims to provide an engine starting assisting system capable of starting an engine even if an engine ECU is reset as the battery voltage becomes less than a reset level when starting the engine. The system includes an auxiliary ECU having a voltage booster, an engine ECU, a starter relay, and a starter that allows the engine to be started when a current flows through the starter relay. The auxiliary ECU and the engine ECU are powered by the starter battery and supply current to the starter relay. When the engine ECU resets the supply of the current to the starter relay because of a voltage drop of the starter battery during starting of the engine, the auxiliary ECU increases the voltage supplied from the starter battery using the booster, thereby supplying requisite current to the starter relay, whereby the engine can be started.
However, such a known arrangement will have a limited ability to compensate fully for reduced performance of the main battery.